[ARM] i16 insert-of-extract to VINS pattern

[ARM] i16 insert-of-extract to VINS pattern

This adds another tablegen fold that converts an i16 odd-lane-insert of
an even-lane-extract into a VINS. We extract the existing f32 value from
the destination register and VINS the new value into it. The rest of the
backend then is able to optimize the INSERT_SUBREG / COPY_TO_REGCLASS /
EXTRACT_SUBREG.

Differential Revision: https://reviews.llvm.org/D95456